Resonant Plasmonic and Vibrational Coupling in a Tailored Nanoantenna for Infrared Detection

A novel resonant mechanism involving the interference of a broadband plasmon with the narrowband vibration from molecules is presented. With the use of this concept, we demonstrate experimentally the enormous enhancement of the vibrational signals from less than one attomol of molecules on individual gold nanowires, tailored to act as plasmonic nanoantennas in the infrared. By detuning the resonance via a change in the antenna length, a Fano-type behavior of the spectral signal is observed, which is clearly supported by full electrodynamical calculations. This resonant mechanism can be a new paradigm for sensitive infrared identification of molecular groups.

Figure 1

Calculation of the near-field intensity 1 nm above the surface of a gold NW (in vacuum, $L=1.31\mu \mathrm{m}$, diameter $D=100\mathrm{nm}$) at the fundamental antenna resonance (${\lambda }_r=3.41\mu \mathrm{m}$). The color scale (shading) indicates the magnitude of the enhancement.

Figure 2

Relative IR transmittance in the spectral region of the fundamental resonance of a gold NW with one ODT monolayer for parallel ($\parallel$) and perpendicular polarization ($\perp$). A ${\mathrm{CaF}}_2$ substrate is used. The broadband plasmonic resonance is observed around ${\lambda }_r\approx 3.6\mu \mathrm{m}(\parallel )$. Inset: Scanning electron micrograph of a gold NW with similar dimensions as used in this study.

Figure 3

Experimental extinction cross sections ${\sigma }_{\mathrm{ext}}$ (normalized to the geometrical cross section ${\sigma }_{\mathrm{geo}}$) for four ODT-coated wires with different $L$ as indicated and $D=100\mathrm{nm}$ on ${\mathrm{CaF}}_2$. The noise level slightly varied depending on the incoming IR intensity. To the right, the zoom to the vibrational signals. Dotted lines: ODT vibration frequencies [6].

Figure 4

Calculated resonance curves for gold NWs with $D=100\mathrm{nm}$, covered with 100 nm ODT layer, for $L$ as indicated, and surrounded by vacuum. The BEM calculations are performed for polarization along the long wire axis. To the right, zoom into the vibrational bands. Dotted lines mark ODT vibrational frequencies.